Metallized ceramic substrates find wide usage for electronic packaging applications, exemplary of which are high density DC/DC converters, power amplifiers, RF circuitry and high current switches. These metallized ceramic substrates take advantage of the electrical conductivities of certain metals and the good thermal conductivities, mechanical strength properties and low electrical conductivities of ceramics. Aluminum nitride (AlN) metallized with copper is particularly favored for advanced applications because of the relatively high oxidation resistance in combination with excellent electrical conductivity of copper and the high thermal conductivity of aluminum nitride.
The state-of-the-art to date in metallizing ceramic substrates consists of either screen-printing usually precious metal inks on the ceramic substrate or depositing very thin layers of vacuum deposited metallization to form the conductive circuit patterns. Both techniques are costly. However, a very large market has developed demanding both less expensive methods and more efficient circuitry.
A thin film circuit on ceramic typically consists of a thin film of metal deposited on a ceramic substrate by one of the vacuum deposition techniques. In these techniques, generally a chromium or molybdenum film, having a thickness of about 0.02 micrometers, acts as a bonding agent for copper or gold layers. Photolithography is used to produce high-resolution patterns by etching away the excess thin metal film. Such conductive patterns may be electroplated up to, typically, 7 micrometers thick. However, due to their high costs, thin film circuits have been limited to specialized applications such as high frequency and military applications where a high pattern resolution is vital.
Another method of producing printed circuits is known as the thick film method. A thick film printed circuit comprises a conductor pattern consisting of a metal, such as silver or gold, and glass frit and/or metal oxide particles fired on a ceramic substrate. Typically, the film has a thickness of about 15 micrometers. Thick film circuits have been widely used and are produced by screen-printing of a circuit pattern with a paste containing the conductive metal powder and a glass fit and/or metal oxide particles in an organic carrier. After printing, the ceramic parts are fired in a furnace to burn off the carrier, sinter the conductive metal particles and fuse the glass. These conductors are firmly bonded to the ceramic by the glass and thus components may be attached to the conductors by soldering, wire bonding and the like.
A disadvantage in using thick film printed circuits is that the conductors have only a 30 to 60 percent of the conductivity of the pure metal. Another disadvantage of thick film technology is that it cannot be applied to aluminum nitride substrates because of a close to zero adhesion of glass to aluminum nitride. In general, there is a need for the higher electric and thermal conductivity attainable by pure metal and aluminum nitride in order to provide the necessary conductive paths for higher density circuits or greater power carrying and thermal dissipation capabilities.
Attempts also have been made in the past to directly bond pure metal conductors to ceramic substrates including alumina, beryllia and aluminum nitride in order to achieve higher conductivity for ceramic-based circuit patterns. See, for example, D. A. Cusano et. al. U.S. Pat. No. 3,994,430 and G. L. Babcock et. al. U.S. Pat. No. 3,766,634 and K. W. Paik et. al. U.S. Pat. No. 5,418,002, which generally describe a process, according to which an oxidized copper foil is juxtaposed on a preconditioned (mostly through surface oxidization) ceramic substrate and heat-bonded thereto utilizing a copper-copper oxide eutectic that is formed during the bonding process at temperatures between 1065 and 1075 degrees Celsius. However, this method suffers from many technological problems, most serious of which is entrapment of gases that cause blistering between the metal layer and the substrate leading to poor yields and high production costs. Also, in order to produce a well-adhered copper foil with minimum blisters, the process parameters have to be controlled very closely, which is hard to do under commercial operating conditions
Other methods tried in attempt to produce reliable metallization of ceramics with pure metal conductors, such as copper, were disclosed in a series of patents and proposed processes that utilize electroless deposition of an initial conductive metal layer onto the ceramic substrate. U.S. Pat. No. 3,690,921 to Elmore involves the use of molten sodium hydroxide to etch a ceramic surface. In this process, following etching the sodium hydroxide is rinsed from the ceramic surface with water, the ceramic surface neutralized with dilute sulfuric acid and rinsed again before sensitizing with a stannous chloride solution, rinsing and seeding with a palladium chloride solution, to catalyze the surface for electroless metal plating. Although the process of Elmore provides good surface coverage it reached only limited acceptance for commercial production because of low adhesion, primarily because the alkaline surface treatment weakens the ceramic surface structure. U.S. Pat. No. 4,428,986 to Schachameyer discloses a method for direct autocatalytic plating of a metal film on a beryllia substrate. However, the method of this patent was able to achieve only 1.7 MPa bond strength. This bond strength is too low for practical use.
Therefore, prior art processes suffer from high costs, or low electrical and thermal performance of metallization layer, or high rejection rate due to blister-prone substrates, or unacceptably low metallization bond strength and are in general unsatisfactory for commercial product purposes. However, it is desirable for economic and increased circuit density design capability reasons to have metallization adhesion to the substrate that will withstand subsequent technological operations and thermal cycling during substrate use as a part of electronic package. Previous attempts to reproducibly manufacture metallized ceramic substrates, particularly aluminum nitride, only experienced limited success. However, the present invention satisfies the foregoing desires by providing a high thermal and electrical conductivity metal layer having excellent adhesion to ceramic surface. In addition, newly invented process provides the following advantages over the prior art, namely (1) it allows to obtain metallization layer thickness, which can be controllably varied in a wide range from a few micro meters to a few hundreds of micrometers; (2) it allows utilizing thick film printing as a process step where inks without glass or oxide fillers are used resulting in better conductive and thermal properties of thick film metallization layer; (3) it enables metallization of not only planar surfaces, but curved surfaces as-well; (4) it results in a high manufacturing yields of metallized ceramic; (5) it provides an economical way to enable soldering or brazing to the metallized ceramic; (6) it provides an inexpensive way to form micro-pins on ceramic surface for enhanced gas or liquid heat transfer applications.